The present invention relates to a Catharanthus (also called periwinkle) seed, a Catharanthus plant, Catharanthus variety, and a Catharanthus hybrid which contain a male sterility gene. This invention further relates to a method for producing a Catharanthus (F.sub.1) hybrid seed and plants.
Field crops including ornamentals, are bred and produced through methods that take advantage of the plant's method of pollination. Self-pollinated crops, including all existing commercial varieties of Catharanthus, rely on the ability of a flower to transfer functional pollen from its anthers to its stigma, thus resulting in formation of seeds. If a line is true breeding for selected desirable characters, and is self-pollinated, then uniform progenies result that can be marketed as uniform cultivar varieties. Repeated selfing or inbreeding, however, results in genetic weakness, variously described as inbreeding depression.
Male sterility is a condition in plants in which male gametophytic function is prevented, but the potential for female reproduction remains. Based on inheritance patterns, there are two general types of male sterility: 1) genic or nuclear male sterility (GMS); and 2) cytoplasmic male sterility (CMS). Male-sterile mutations provide source material for studies in plant breeding, genetics, reproductive biology, and molecular biology.
An alternative to the self-pollinated Catharanthus varieties are F.sub.1 hybrids. In F.sub.1 hybrid varieties, pollen from an inbred "male" line is used to pollinate an inbred, but genetically different "female" line. The resulting F.sub.1 hybrids are both phenotypically uniform and vigorous. In addition to this hybrid vigor, hybrids also offer opportunities for the rapid and controlled deployment of dominant genes for resistance to diseases and pests. A homozygous dominant gene in one parent of a hybrid will result in all F.sub.1 hybrids expressing the dominant gene phenotype. In ornamentals, certain flower color phenotypes can only be achieved in the heterozygous state of F.sub.1 hybrids.
Much progress has been made in the improvement of horticultural and agronomic crops over the past several decades. Prominent among the methods used has been that of F.sub.1 hybrid seed production. Essentially all corn, tomato, cucumber, and vegetable crops in general, are grown from F.sub.1 hybrid seed. Ornamentals including petunias, geraniums, impatiens, snapdragons, and many others are grown as F.sub.1 hybrids. Within the seed trade industry, F.sub.1 hybrids command the preeminent role because of their superior vigor, uniformity and performance.
The efficient prevention of self-fertilization is a key requirement in F.sub.1 hybrid seed production. In ornamentals, such as petunia or Catharanthus, uniformity is very important since even a small level (such as 1%) of selfs can destroy the commercial value of the F.sub.1 seed crop. Various methodologies are used to prevent selfing in the female. Emasculation, manual removal of anthers prior to anther dehiscence, is the primary means of preventing selfing in soybeans, tomatoes, petunias and numerous other crops. The emasculation operation in ornamentals is both logistically difficult (since a single flower cannot be overlooked and missed) and expensive because of its labor intensiveness. Some crops, such as Catharanthus, are unsuited to commercial emasculation since the flower parts prior to dehiscence are too small for commercial-level manual manipulation. Self incompatibility (SI) has also been utilized as a means of preventing accidental selfing in the female. Besides environmental instability (reversions to self-compatibility dependent on climate), the complexity of the production of inbred lines can be a major drawback in use of self-incompatibility. The absence of SI in any Catharanthus taxa (see Veyret, Candollea 29:297-307; Levy et al., Euphytica 32: 557-564 (1983)) precludes its utilization to prevent self-fertilization in F.sub.1 hybrid seed production. Another method for prevention of accidental selfing includes application of chemical hybridizing agents (see Chia and Ruminski, J. Agric. Food Chem 39: 2072-2076 (1991)) that suppress pollen function; these agents are not used in flower seed production due to their cost, environmental instability, inconsistent and incomplete effectiveness, and their potential toxicity to greenhouse personnel. No male chemical sterilants are currently registered for greenhouse use on ornamental crops.
Dioecism, a naturally occurring phenomenon in some species wherein individual plants produce male or female parts, but not both, effectively prevents self-pollination. Unfortunately, few species of economic consequence are dioecious. Dioecy is not present in any Catharanthus species, nor has it been reported elsewhere in the Apocynaceae family.
Male sterility, both naturally occurring and artificially induced, is another means of achieving prevention of self-pollination in plants, aside from manual emasculation. In male sterility (MS) systems, absence of pollen in normally hermaphroditic flowers precludes the possibility that flowers will pollinate themselves. Without access to pollen, sexual fusion of the male and female gametes that would normally lead to seed development does not occur; the end consequence is that no "self" seed (i.e. seed arising from self-pollinations) is produced.
In higher plants, two major types of male sterility can be distinguished according to their genetic control. Nuclear male sterility (NMS), sometimes referred to as genic, genetic, or Mendelian sterility is controlled by genes carried and expressed within the nucleus of cells. Inheritance of NMS typically follows normal Mendelian segregation patterns. In contrast, cytoplasmic male sterility (CMS) is governed by cytoplasmic factors, principally the mitochondrial genome; inheritance of CMS does not follow Mendelian patterns and instead, is associated with maternal transmission of mitochondrial components from generation to generation. Neither NMS or CMS is known to occur in any Catharanthus species, nor are they known to occur naturally elsewhere in the Apocynaceae family.
Catharanthus species rely heavily on self-pollination for natural seed production. Lack of self-incompatibility, dioecy, or male sterility in any Catharanthus species indicates that self-pollination and self-set seed is an important and universal component of the reproductive biology of periwinkle species. For effective commercial production of F.sub.1 hybrid Catharanthus cultivars, a more reliable system to produce F.sub.1 hybrid seed is desirable. Use of a reliable male sterility gene in Catharanthus, if available, would result in efficient commercial production of hybrid Catharanthus.